The present invention relates to magnetic tracking systems of the type wherein a magnetic field is established in a relevant work area, and one or more magnetic field sensors are operated to sense values of a local magnetic field and are processed to determine the position of a tool, instrument or other identified object. In general, such systems operate using a field generating element or assembly, and a field sensing element or assembly, often in conjunction with other position determining elements, such as robotic stepper or optical tracker assemblies to track the relative changes in position between one or more fixed points or structures in the physical environment or associated images, and one or more moving or non-visible points or structures in the work arena.
Magnetic field generating or sensing assemblies for tracking may be implemented in various ways, with conventional analog wire coils forming current loops or paths, or with semiconductor or microlithographically-formed conductive leads or circuit board traces forming current paths, arranged in an appropriate geometry to generate or sense the desired field components. There may be a symmetry or duality between the generating or sensing elements. Thus, for example in many cases it is possible to have a small multi-coil array that generates a spatially distributed magnetic field and a similar or even identical array that senses the field so generated. Small coils offer the prospect of generating, to a close approximation, dipole fields, although small size may limit the attainable field strength or the achievable level of detection signal amplitude. The generating and sensing constructions may alternatively employ different scales, for example, with relatively large and/or high current coils to establish magnetic field components along different axes, and smaller, or more localized coil assemblies for sensing field values. Smaller coils, whether for sensing or generating, may, for example, be fastened to the body, or attached to workplace or surgical instruments, or to catheters or other body-inserted devices, to sense the magnetic field and track position of the attached structure.
In general, it is the aim of such magnetic tracking assemblies to define the spatial coordinates (e.g., position and orientation coordinates, either absolute or relative) where the movable magnetic assembly is located at a given instant in time. It is therefore necessary to characterize the magnetic field distribution or signal values with some degree of accuracy, and also necessary to accurately detect the field. The field distribution may be determined by a combination of field modeling and empirical field mapping. The latter, for example, may be carried out as a calibration or an initialization step, and may be performed to correct a theoretical field distribution for the presence of interfering structures. In any case, the spatial coordinates are generally computed for one magnetic assembly (transmitter or sensor) with respect to the other magnetic assembly (sensor or transmitter). Typically, one of these assemblies is itself fixed.
One area in which magnetic tracking has been useful is the area of image guided surgery. Typical image guided surgical systems acquire a set of images of an operative region of a patient's body, and track a surgical tool or instrument in relation to one or more sets of coordinates, e.g., spatial coordinates of the surgical work arena, the coordinates of the images themselves, or a target feature of the patient's anatomy. At the present time, such systems have been developed or proposed for a number of surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac or other interventional radiological procedures and biopsies. Such procedures may also involve pre-operative or intraoperative x-ray images being taken to correct the position of and refine the display of, or otherwise navigate a tool or instrument involved in the procedure in relation to anatomical features of interest.
Several areas of surgery have required very precise planning and control. Such tracking is useful for the placement of an elongated probe, radiation needle, fastener or other article in tissue or bone that is internal or is otherwise positioned so that it is difficult to view directly. For brain surgery, stereotactic frames may be used to define the entry point, probe angle and probe depth to access a site in the brain. Furthermore, many of the foregoing techniques may be used in conjunction with previously-compiled three-dimensional diagnostic images such as MRI, PET or CT scan images to provide accurate tissue coordinates to which the tracked physical elements may be referenced. Such systems offer valuable improvements for procedures such as placement of pedicle screws in the spine, where visual and fluoroscopic imaging directions cannot capture the axial view that would be required to safely orient the insertion path through bony material close to the spinal cord.
When used with existing CT, PET or MRI image sets, the previously recorded diagnostic image sets, by virtue of their controlled scan formation and the spatial mathematics of their reconstruction algorithms, define a high precision three dimensional rectilinear coordinate system. However, even when provided with such reference images it is necessary to correlate and fit available measurement and views and anatomical features visible from the surface, with features in the diagnostic images and with the external coordinates of the tools being employed. This is often done by providing implanted fiducials, by adding external visible or trackable markers that may be imaged, and by enabling a surgeon or radiologist to use a keyboard or mouse to identify fiducials or features in the various images, and thus map common sets of coordinate registration points in the different images. Given a fit of spatial points to image points, software may then track changing positions in an automated way (for example, simply transforming the coordinates that are output by an external coordinate measurement device, such as a suitably programmed off-the-shelf optical tracking assembly.) Instead of correlating image positions of a set of imageable fiducials appearing in fluoroscopic or CT images, such systems can also operate with simple optical tracking, employing an initialization protocol wherein the surgeon touches or points at a number of bony prominences or other recognizable anatomic features in order to define the external coordinates in relation to the patient anatomy and to initiate software tracking of those features.
For such applications, electromagnetic tracking offers the advantage that its position-defining field, a magnetic field, penetrates the body without attenuation or change so that tracking may continue during a surgical procedure, unimpaired by the blocking that occurs with visible light trackers (e.g., due to operating room personnel moving into positions that obstruct the line-of-sight paths required by optical trackers). Optical or ultrasonic tracking, by contrast, may require a larger or excess number of line-of-sight paths, and corresponding transponders and/or detectors to assure that triangulation is always possible despite occluded pathways. The body-penetrating electromagnetic fields also allow one to track locations or movements inside the body with minimal resort to the fluoroscopic or ultrasound techniques normally required for visualization.
Among electromagnetic tracking techniques, several principal approaches are known. In one, relatively large Helmholtz coils establish well defined and highly uniform independent magnetic field components or gradients along each of the X, Y and Z axes in a work arena, and the static field components are detected by a localized detector to determine position coordinates. This approach has been proposed, for example, for cranial surgery, where such coils may define a suitably localized region encompassing the entire operative arena. Another principal approach involves using time-varying dipole fields e.g., dipole fields established by driving field-generating coils with an AC current signal. While the latter approach offers some processing advantages (such as being able to synchronously demodulate induced signals, and thus cumulate detected signal values to enhance sensitivity, and also the ability to establish the X, Y and Z field components at different frequencies so that detected sensor output signals may be separated or even demodulated simultaneously), it has the disadvantage that varying magnetic fields induce eddy currents in conductive structures found within the field. Induced currents themselves then generate secondary magnetic fields, thus introducing distortions into the expected or calibrated field distribution. Conductive or metal structures are in fact commonly present in a tracking environment, whether it be an avionics, surgical or industrial tracking environment.
The latter problem has historically been addressed by the observation that for a fixed metal disturber located at least twice as far from the field generator as is the field sensor, the induced disturbance will be low, e.g., under one percent, so that by restricting the tracked arena to a region sufficiently removed from the disturbing structure, accuracy may be achieved. However, this approach may be unrealistically restrictive for certain applications, including some image guided surgery applications, where highly disturbing equipment (such as the imaging assembly of a fluoroscope) is necessarily placed as close as possible to the work arena in which tracking is to occur. Another common alternative approach would be to map the disturbed fields or detected signal levels that occur close to the distorters present in the work arena, so that a processor can more accurately determine coordinates from the run time field values or the induced signals detected by a sensor. However, as a practical matter, such mapping is not only likely to require a time-consuming preliminary set-up operation, but may require that the set-up be carried out afresh for each new arrangement of operating room equipment that introduces different distortions, i.e., when equipment is changed or moved.
In addition to the practical problem of accurately detecting the field, there is the theoretical problem of converting the signal measurements into position and orientation coordinates.
On a fundamental level, the task that must be addressed by any electromagnetic tracking system is to computationally determine a unique position from the various measured parameters (typically induced voltages indicative of field strength). Often the relevant equations have several solutions, and care must be taken to operate within a single-valued domain (typically a hemisphere, quadrant or octant), thus limiting the selection of the initial generator or receiver fixed locations to establish a sufficiently well behaved field region, and restricting the allowable work arena in relation thereto, or else providing additional, or extrinsic data inputs to resolve ambiguities or refine computations. Beyond encountering multi-valued coordinate solutions, the accuracy of the coordinate determination processing may depend quite critically on the relative positions of generating and sensing elements. Processing equations may break down or solutions may become poorly defined as the sensing or generating element approaches or crosses a particular plane or axis, or one of its pitch, yaw or roll angle coordinates approaches 0 or 90 degrees, becomes too acute, or becomes too oblique.
Another practical problem stems from the dynamic range of the sensed signals, which varies greatly with distance/position, and may drop to quite low levels with increasing distance from the transmitter, or with decreasing size of the generating or sensing coils. Often, as a field unit moves to different positions, different gains must be determined on the fly, or additional gain stages must be used in order to obtain adequate signal values. This introduces some complexity and potential for error in normalizing the signals to accurately fit together readings from two different or even closely contiguous regions.
Thus, while magnetic tracking offers certain significant advantages, particularly for surgical applications, there remains a need for improved systems and processing to enhance accuracy of coordinate determinations.
Accordingly, it would be desirable to provide an electromagnetic tracker of enhanced accuracy.
It would also be desirable to provide an electromagnetic tracker having enhanced immunity to common field distortions.